Hot press-formed member having excellent crack propagation resistance and ductility, and method for producing same

ABSTRACT

Provided is a hot press-formed member having excellent crack propagation resistance and ductility. The hot press-formed member includes: a base steel sheet and a zinc or zinc alloy plating layer on at least one surface of the base steel sheet. The base steel sheet contains, by wt %, carbon (C): 0.08-0.30%, silicon (Si): 0.01-2.0%, manganese (Mn): 3.1-8.0%, aluminum (Al): 0.001-0.5%, phosphorus (P): 0.001-0.05%, sulfur (S): 0.0001-0.02%, nitrogen (N): 0.02% or less, and a balance of iron (Fe) and other impurities. The hot press-formed member comprises 1-30 area % of retained austenite as a microstructure, and a Mn(wt %)/Zn(wt %) content ratio in an oxide layer of 0.5-1.2 μm in a thickness direction from a surface layer of the plating layer is 0.1 or more.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application is a Continuation patent application of U.S. patentapplication Ser. No. 16/315,767, filed on Jan. 7, 2019, which is theU.S. National Phase under 35 U.S.C. § 371 of International PatentApplication No. PCT/KR2017/007328, filed on Jul. 17, 2017, which in turnclaims the benefit of Korean Application No. 10-2016-0086248, filed onJul. 7, 2016, the disclosures of which applications are incorporated byreference herein.

TECHNICAL FIELD

The present disclosure relates to a hot press-formed member suitable foruse as an automotive material, and more specifically, to a hotpress-formed member having excellent crack propagation resistance andductility, and to a method for producing the same.

BACKGROUND ART

Recently, hot press-formed members have been widely used as automotivestructural members for purposes such as reducing the weight of a vehicleand improving the fuel efficiency of a vehicle, and a great deal ofresearch has been conducted for such purposes.

For example, the invention disclosed in Patent Document 1 may provide amember having an ultra-high strength in the range of 1,500 MPa orgreater in tensile strength, by hot press forming and rapid cooling analuminum-plated steel sheet after heating the same to 850° C. or higher.In addition, due to the formation of an Al—Fe alloy layer, a steel sheethaving excellent corrosion resistance may be provided.

However, according to the patent document, the main phase of a coatedsteel sheet is formed by aluminum, so it may be difficult to exploit thesacrificial effects of aluminum, and ductility cannot be improved.

In this context, Patent Document 2 discloses a technique to effectivelyachieve the sacrificial effects of a hot press-formed member, thetechnique characterized in that after a Zn-plated steel material isheated at 880-980° C. for 6-15 minutes, a buffer layer is formed atboundaries between a base steel sheet and a plating layer.

However, in this case, as a heating temperature becomes 880° C. orgreater, spot weldability deteriorates due to zinc oxides formed on azinc surface, and ductility cannot be improved, and due to an inferiorcrack propagation resistance, it is difficult to control the depths ofmicrocracks to be 10 μm or less.

For hot press-formed members to be suitable for use as impact-resistantmembers of vehicles, the hot press-formed members not only requireultra-high strength, but also need to have improved corrosion resistanceand crack propagation resistance, as well as fatigue properties andcrashworthiness, and therefore, it is necessary to develop a hotpress-formed member that satisfies the aforementioned properties.

-   (Patent Document 1) U.S. Pat. No. 6,296,805-   (Patent Document 2) Korean Laid-Open Patent Application No.    2014-0035033

DISCLOSURE Technical Problem

An aspect of the present disclosure is to provide a hot press-formedmember having excellent crack propagation resistance and ductility,which may be suitably used for automotive structural members orreinforcing materials that require corrosion resistance andcrashworthiness, and a method for producing the same.

Technical Solution

An aspect of the present disclosure provides a hot press-formed memberhaving excellent crack propagation resistance and ductility.

According to an aspect of the present disclosure, the hot press-formedmember comprising a base steel sheet and a plating layer comprising Znor Zn alloy plating layer on the base steel sheet,

wherein a Mn(wt %)/Zn(wt %) content ratio measured at a point 1 μm froma surface of the plating layer is 0.1 or more,

wherein a maximum crack depth penetrating the base steel sheet from aninterface between the base steel sheet and the plating layer is 10 μm orless.

Advantageous Effects

According to the present disclosure, a hot press-formed member havingexcellent ductility and crack propagation resistance while having atensile strength of 1,300 MPa or more, may be provided.

In addition, the hot press-formed member of the present disclosure maybe suitably used for automotive structural members or reinforcingmaterials that require corrosion resistance and crashworthiness.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a hat-shaped hot press-formed memberaccording to an embodiment of the present disclosure.

FIG. 2 is photographs of cross-sections of Inventive Example andComparative Example according to an embodiment of the presentdisclosure.

BEST MODE FOR INVENTION

In manufacturing a hot press-formed member using a coated steel sheet,the present inventors have investigated on improving ductility and crackpropagation resistance, as well as strength, of the hot press-formedmember.

As a result, the present inventors have confirmed that when heating andhot forming temperatures, post-heating process conditions, and the like,are optimized, a hot press-formed member having desired properties maybe provided.

In a conventional hot forming method using a zinc or zinc alloy platingmaterial, the plated material is heated at a high temperature and formedat the high temperature, and when heating at the high temperature, alarge quantity of soft zinc oxides are formed on the surface of theplated material, which generate microcracks when forming, so fatigueproperties and bending properties of the hot press-formed memberdeteriorate. In addition, coarse-grained prior austenite may be formed,and it may be difficult to achieve sufficient retained austenite as amicrostructure of a final hot press-formed member, so the hotpress-formed member thus produced may be unsuitable for use ascrashworthy and impact-absorbing members.

In light of the above, when manufacturing a hot press-formed memberusing a zinc or zinc alloy plating material, in the present disclosure,a content of manganese (Mn) in the plated material is increased, so whenheating to a high temperature for hot forming, hard manganese oxides maybe formed on the surface relatively more than zinc oxides, resulting inan increased number of microcracks on a wall part of the plating layer(for example, the observed surface shown in FIG. 1 ), the part to whichshear deformation is applied when forming, thus distributing sheardeformation stress. In addition, by having a hot forming temperature ina lower temperature region than in the conventional method, the presentdisclosure may improve microcrack propagation resistance. In addition,the present disclosure may reduce a prior austenite grain size andensure a sufficient retained austenite phase to improve crashworthyproperties. In addition, through a post-heat treatment process(referring to a tempering process in the present disclosure), thepresent disclosure may further stabilize the retained austenite phase,further improve yield strength, and further improve crashworthiness byreducing dislocation density within martensite.

Hereinbelow, the present disclosure will be more fully described indetail.

An aspect of the present disclosure provides a hot press-formed memberhaving excellent crack propagation resistance and ductility, comprisinga base steel sheet and a zinc or zinc alloy plating layer on at leastone surface of the base steel sheet.

The base steel sheet may be a hot-rolled steel sheet or cold-rolledsteel sheet containing a certain percentage of manganese (Mn), and moreparticularly, the base steel sheet preferably contains, by wt %, carbon(C): 0.08 to 0.30%, silicon (Si): 0.01 to 2.0%, manganese (Mn):3.1-8.0%, aluminum (Al): 0.001-0.5%, phosphorus (P): 0.001-0.05%, sulfur(S): 0.0001-0.02%, and nitrogen (N): 0.02% or less.

Hereinbelow, reasons for limiting the alloy composition of the basesteel sheet will be described in detail, and the content of eachcomponent is provided in wt % unless otherwise specified.

C: 0.08 to 0.30%

Carbon (C) is an essential element for improving the strength of a hotpress-formed member, and in the present disclosure, is useful forsecuring retained austenite that needs to be formed to achieve excellentductility, and therefore, should be added in a proper amount.

If the content of carbon (C) is less than 0.08%, it may be difficult toachieve sufficient strength and ductility, so it may be preferable thatcarbon (C) be added in an amount of 0.08% or more. However, if thecontent of carbon (C) exceeds 0.30%, the cold rolling property of ahot-rolled steel sheet may be markedly lowered in a cold rolling processbecause the strength of the hot-rolled steel sheet may be too high, andspot weldability may be markedly lowered as well.

Accordingly, in the present disclosure, it may be preferable that thecontent of carbon (C) be limited to 0.08-0.30%.

Si: 0.01 to 2.0%

Silicon (Si) is added to remove oxygen in a steel making process,suppress the formation of carbides, which markedly affects the strengthof a hot press-formed member, and is an element useful for securingretained austenite by concentrating carbon (C) along grain boundaries oflath martensite after martensite is formed when hot forming.

If the content of silicon (Si) is less than 0.01%, the above-describedeffects may not be achieved, and it may be difficult to achieve a highdegree of cleanliness of steel, moreover, controlling the content ofsilicon (Si) may be costly. Meanwhile, the content of silicon (Si)exceeding 2.0% may markedly lower coatability when plating with zinc ora zinc alloy, and thus may not be preferable.

Accordingly, in the present disclosure, it may be preferable that thecontent of silicon (Si) be limited to 0.01-2.0%. It may be morepreferable that silicon (Si) be added in an amount of 1.5% or less, andeven more preferable that silicon (Si) be added in an amount of 0.7% orless.

Mn: 3.1-8.0%

Manganese (Mn) is an extremely important element in the presentdisclosure. Manganese (Mn) serves to provide solid solutionstrengthening effects, serves to lower an Ac3 temperature (thetemperature at which 100% transformation to austenite takes place duringheating), and serves to lower an Ms temperature (the temperature atwhich martensitic transformation begins). Particularly, in the presentdisclosure, the hot forming is performed in relatively low temperatureregions, and when the heated material, after being extracted from theheating furnace, is cooled to a temperature for the hot forming, themanganese (Mn) suppresses the formation of ferrite which causesdegradations in strength, and therefore, needs to be added in asufficient amount.

If the content of manganese (Mn) is less than 3.1%, the above-describedeffects may not be sufficiently achieved, and if the content ofmanganese (Mn) exceeds 8.0%, the strength of the steel sheet beforeperforming the hot forming becomes excessively high, thus degradingworkability, and even when the temperature of the material is increased,hot forming properties may deteriorate, and the manufacturing costs ofalloy components may be increased, thus lowering manufacturingcompetitiveness.

Accordingly, in the present disclosure, it may be preferable that thecontent of manganese (Mn) be limited to 3.1-8.0%, and more preferably,to 3.5-8.0%.

Al: 0.001-0.5%

Aluminum (Al) is an element effective in removing oxygen in a steelmaking process, along with the silicon (Si), to increase a degree ofcleanliness of steel.

If the content of aluminum (Al) is less than 0.001%, the above-describedeffect may be difficult to achieve, and if the content of aluminum (Al)exceeds 0.5%, the Ac3 temperature may be excessively elevated, so that aheating temperature for the hot forming may need to be increased evenfurther.

Accordingly, in the present disclosure, it may be preferable that thecontent of aluminum (Al) be limited to 0.001-0.5%.

P: 0.001-0.05%

Phosphorus (P) exists as an impurity in steel, and high manufacturingcosts may be incurred to keep the content of phosphorus (P) less than0.001%, whereas a content of phosphorus (P) exceeding 0.05% may markedlydecrease the weldability of a hot press-formed member, and thus may notbe preferable.

Accordingly, in the present disclosure, it may be preferable that thecontent of phosphorus (P) be limited to 0.001-0.05%, and morepreferably, to 0.02% or less.

S: 0.0001-0.02%

Sulfur (S) exists as an impurity in steel, and is an element negativelyaffecting the ductility, impact properties, and weldability of a hotpress-formed member, and thus, it may be preferable that the content ofsulfur (S) be limited to 0.02% or less. However, manufacturing costs maymarkedly increase to keep the content of sulfur (S) less than 0.0001%,so it may be preferable that a lower limit of the content of sulfur (S)be limited to 0.0001%.

Accordingly, in the present disclosure, it may be preferable that thecontent of sulfur (S) be limited to 0.0001-0.02%, and more preferably,to 0.01% or less.

N: 0.02% or less

Nitrogen (N) is an element contained as an impurity in steel, and if thecontent of nitrogen (N) exceeds 0.02%, a slab may become prone tocracking during continuous casting, and impact properties maydeteriorate as well.

Accordingly, in the present disclosure, it may be preferable that thecontent of nitrogen (N) be limited to 0.02% or less. However,manufacturing costs may drastically increase for keeping the content ofnitrogen (N) less than 0.0001%, so it may be preferable that a lowerlimit of the content of nitrogen (N) be limited to 0.0001%.

The base steel sheet of the present disclosure, in addition to theabove-described alloy composition, may further contain the followingelements, which will be described below. Particularly, the base steelsheet may further contain, preferably, at least one group selected fromthe following groups (1) to (4), and reasons for limiting the alloycomposition of each group will be fully described in detail hereinbelow.

(1) At least one of chromium (Cr) and molybdenum (Mo), a total contentthereof being 0.001-2.0%.

(2) At least one among titanium (Ti), niobium (Nb), and vanadium (V), atotal content thereof being 0.001-0.2%.

(3) At least one of copper (Cu) and nickel (Ni), a total content thereofbeing 0.005-2.0%.

(4) Boron (B): 0.0001-0.01%.

The total content of at least one of chromium (Cr) and molybdenum (Mo):0.001-2.0%

Chromium (Cr) and molybdenum (Mo) are elements useful for achievingstrength and grain refinement through improving hardenability andprecipitation strengthening effects. If the total content of at leastone of chromium (Cr) and molybdenum (Mo) is less than 0.001%, theabove-described effects may not be achieved, however, the total contentthereof exceeding 2.0% may saturate these effects, and also causeweldability to deteriorate and manufacturing costs to increase, and thusmay not be preferable.

Accordingly, in the present disclosure, it may be preferable to limitthe total content of at least one of chromium (Cr) and molybdenum (Mo)added, to 0.001-2.0%.

The total content of at least one among titanium (Ti), niobium (Nb), andvanadium (V): 0.001-0.2%

Titanium (Ti), niobium (Nb), and vanadium (V) have the effect ofimproving the strength of a hot press-formed member by forming fineprecipitates, as well as the effect of stabilizing retained austeniteand improving impact toughness by inducing grain refinement. If thetotal content of at least one among titanium (Ti), niobium (Nb), andvanadium (V) is less than 0.001%, the above-described effects may bedifficult to achieve; however, a total content thereof exceeding 0.2%may saturate these effects, and incur an increase in the cost forferro-alloys, and thus may not be preferable.

Accordingly, in the present disclosure, it may be preferable to limitthe total content of at least one among titanium (Ti), niobium (Nb), andvanadium (V) to 0.001-0.2%.

The total content of at least one of copper (Cu) and nickel (Ni):0.005-2.0%

Copper (Cu) may be added as an element improving strength by formingfine precipitates, and nickel (Ni) may be optionally added, as needed,since hot brittleness may be caused when copper (Cu) is added alone. Ifthe total content of at least one of copper (Cu) and nickel (Ni) is lessthan 0.005%, the above-described effects may be difficult to achieve,however, the total content thereof exceeding 2.0% may incur an excessiveincrease in the cost.

Accordingly, in the present disclosure, it may be preferable to limitthe total content of at least one of copper (Cu) and nickel (Ni) addedto 0.005-2.0%.

Boron (B): 0.0001-0.01%

The addition of even a small amount of boron (B) may improvehardenability, and boron (B) segregates along prior-austenite grainboundaries to suppress embrittlement of a hot press-formed member causedby phosphorus (P) and/or sulfur (S) segregating along grain boundaries.If the content of boron (B) is less than 0.0001%, the above-describedeffects may be difficult to achieve; however, the content of boron (B)exceeding 0.01% may saturate these effects, causing brittleness when hotrolling.

Accordingly, in the present disclosure, it may be preferable to limitthe content of boron (B) added to 0.0001-0.01%, and more preferably, to0.005% or less.

Other than the above-described alloy components, the remainder componentis iron (Fe). However, since in a conventional manufacturing process,inevitable impurities may be introduced from raw materials orsurrounding environments, these impurities should not be excluded. Sincethese impurities are well known to anyone skilled in the conventionalmanufacturing process, they will not be described in the presentdescription.

The hot press-formed member of the present disclosure comprises aplating layer on at least one surface of a base steel sheet having theabove-described composition, wherein the plating layer is a zinc or zincalloy plating layer, and may be, for example, a zinc (Zn)-plated layer,a zinc (Zn)-iron (Fe) plated layer, a zinc (Zn)-aluminum (Al) alloyplated layer, or a zinc (Zn)-aluminum (Al)-magnesium (Mg) alloy platedlayer. However, the plating layer is not limited thereto.

The hot press-formed member of the present disclosure may preferablycomprise 1-30 area % of retained austenite in microstructures thereof.

If a phase fraction of the retained austenite is less than 1%, a desiredlevel of ductility may not be achieved. However, the phase fraction ofretained austenite exceeding 30% may guarantee excellent ductility,however, this may require carbon (C) or manganese (Mn) to be added inlarger amounts, significantly degrading the spot weldability and impactproperties of a hot press-formed member, and thus may not be preferable.

Meanwhile, the phase fraction of retained austenite, as will bedescribed below, may be controlled through hot forming conditions, andwhen hot forming and the subsequent cooling are performed in theconditions proposed in the present disclosure, 1-20 area % of retainedaustenite phase may be formed, and when a tempering process after thecooling is further performed, 1-30 area % of retained austenite phasemay be guaranteed.

The phase fraction of retained austenite may be derived by calculatingthe area under an austenite peak, through an X-ray diffraction test.

The remaining part other than the remained austenite phase may bepreferably at least one of martensite and bainite, or at least one oftempered martensite and bainite.

In addition, the hot press-formed member of the present disclosure haspreferably a prior austenite grain size (PAGS) of 10 μm or less. If thePAGS exceeds 10 μm, the amount of impurities per unit area, such as P orS segregating along grain boundaries, increases during a heat treatment,and microcrack propagation may be difficult to suppress, so crashworthycharacteristics may deteriorate.

Accordingly, it may be preferable that the PAGS be 10 μm or less, morepreferably, 7 μm or less, and even more preferably, 5 μm or less.

The smaller the PAGS, the smaller the amount of impurities segregatingalong grain boundaries, and the easier it is to suppress the propagationof microcracks, however, manufacturing costs may markedly increase forkeeping the PAGS under 0.1 μm, and thus, it may be preferable to limitthe lower limit thereof to 0.1 μm.

Further, in the hot press-formed member of the present disclosure, itmay be preferable that a ratio of manganese to zinc (Mn (wt %)/Zn (wt%)) in an oxide layer of 0.5-1.2 μm in a thickness direction from thesurface layer of a plating layer be 0.1 or more.

As described above, by increasing the amount of manganese oxides in theoxide layer in a surface layer region of the plating layer, it ispossible to effectively suppress the generation of microcracks in a sidesurface portion (for example, the observed surface shown in FIG. 1 )which mainly receives the effects of shear transformation during hotforming.

An upper limit of the ratio of manganese to zinc (Mn/Zn) in the oxidelayer is not limited to any particular value, however, for the upperlimit to exceed 2.0, the content of manganese (Mn) needs to exceed 8%,in which case manufacturing costs may increase and operationality maysuffer. Accordingly, it may be preferable to limit the upper limit ofthe ratio of manganese to zinc to 2.0.

Hereinbelow, a method for producing a hot press-formed member, anotheraspect of the present disclosure, will be described in detail.

First, it may be preferable to prepare a base steel sheet containing theabove-described alloy composition and then, to plate the base steelsheet with zinc or a zinc alloy to produce a coated steel sheet.

The base steel sheet, as described above, may be a hot-rolled steelsheet or a cold-rolled steel sheet, and may be produced by the methoddescribed below.

The hot-rolled steel sheet may preferably be produced by re-heating asteel slab having the above-described alloy components, and subjectingthe steel slab to a finish hot-rolling process and a coiling process.

For example, it may be preferable that in the re-heating process, thesteel slab be heated to a heating temperature of 1,000-1,300° C. If theheating temperature is less than 1,000° C., it may be difficult tohomogenize the structure and composition of the slab, however, theheating temperature exceeding 1,300° C. may give rise to excessiveoxidation and equipment degradation, and thus, may not be preferable.

Subsequently, the reheated slab may be finish hot rolled to produce ahot-rolled steel sheet. The finish hot rolling may be performedpreferably at a temperature of Ar3 to 1,000° C., and if the finish hotrolling temperature is Ar3 or less, the rolling may occur in a two-phaseregion, and thus, it may be difficult to control the structure ofsurface mixed grains and the shape of a steel sheet. If the finish hotrolling temperature exceeds 1,000° C., the grains of the hot-rolledmaterial may become coarse.

Subsequently, the hot-rolled steel sheet produced above may be coiledinto the shape of a coil, preferably at a temperature of 750° C. or lesswhile exceeding Ms. If the coiling temperature is Ms or less, thestrength of the hot-rolled material may become excessively high, thusincreasing the load for subsequently performed cold rolling. If thecoiling temperature exceeds 750° C., the oxidation of grain boundariesof the hot-rolled material may be excessive, thereby causing picklingproperties to deteriorate.

The cold-rolled steel sheet may be produced, preferably by pickling,cold rolling, and annealing the hot-rolled steel sheet produced above.

At this point, the cold rolling is to manufacture a cold-rolled steelsheet having a desired thickness, and may be preferably performed at areduction ratio of 10-80%.

The cold-rolled steel sheet obtained above may be preferably annealed ina continuous annealing furnace to lower the strength thereof. Thestrength of the steel sheet may be effectively lowered by performingcontinuous annealing at a temperature of 600-900° C. for 1-1000 seconds.

Also, the hot-rolled steel sheet before or after performing the picklingtreatment, prior to performing the cold rolling, may be subjected to abatch annealing process. This is to lower the strength of the hot-rolledmaterial, and may preferably be performed at a temperature of 400-700°C. for 1-100 hours, in particular.

It may be preferable that the hot-rolled steel sheet or cold-rolledsteel sheet produced as described above be subjected to a platingprocess to produce a coated steel sheet.

The plating process is not limited to any particular process, but may bezinc or zinc alloy plating. In particular, when the base steel sheet isa hot-rolled steel sheet, hot dip zinc plating, hot dip zinc alloyplating, or electrical zinc plating may all be applicable. However, whenthe base steel sheet is a cold-rolled steel sheet, it may be preferablethat electrical zinc plating be performed. For example, the hot dip zincalloy plating may be zinc-aluminum alloy plating,zinc-aluminum-manganese alloy plating, or the like. Such platingprocesses may be performed to their respective conventional conditions,so such conditions are not limited to particular conditions.

Further, prior to the plating process, metal coating with iron (Fe),nickel (Ni), or the like may be performed for the purpose of improvingplating adhesion.

As described above, the coated steel sheet obtained according to anembodiment of the present disclosure may be hot formed to produce a hotpress-formed member.

First, it may be preferable that the coated steel sheet be heated to andmaintained at a temperature equal to or greater than a certaintemperature. For example, it may be preferable that the coated steelsheet be introduced in a heating furnace and heated to a temperatureequal to or greater than Ac3, which is equal to or greater than a singlephase of austenite, at a heating rate of 1-1,000° C./s, and bemaintained for 5-10,000 seconds.

More preferably, when heating at the above-described heating rate, amaximum heating temperature is preferably within a temperature rangefrom more than Ac3+10° C. to less than Ac3+200° C. When the maximumheating temperature is Ac3+10° C. or less, even if heated to atemperature equal to or greater than Ac3, local regions may exist wherecarbon (C) and/or manganese (Mn) are not segregated, thus, completetransformation to austenite may not take place, causing prior-ferrite toremain, and thus, yield strength may not be sufficiently improved. Inaddition, springback of the hot press-formed member may be excessive,making it difficult to achieve shape fixability. However, when themaximum heating temperature is Ac3+200° C. or more, it may be easy toachieve sufficient strength and shape fixability, but spot weldabilitymay deteriorate due to an excessive formation of oxides on the surfaceof the plating layer.

Accordingly, it may be preferable that the coated steel sheet bemaintained in the above-described temperature regions, however, if themaintaining time is less than 5 seconds, complete austenization may nottake place, and also, it may be difficult to achieve a Mn/Zn contentratio of 0.1 or more in surface oxides. The maintaining time exceeding10,000 seconds may reduce spot weldability due to an excessive formationof oxides on the surface.

It may be preferable that the coated steel sheet, heated and maintainedas described above, after being extracted from the heating furnace, becooled to a hot forming temperature. At this point, it may be preferablethat a cooling rate be controlled to be less than 50° C./s, since inorder to keep the cooling rate to be 50° C./s or more, separate coolingequipment may be required, undesirably incurring an increase inmanufacturing costs. Therefore, it may be more preferable that thecooling be performed at a cooling rate less than 30° C./s, and even morepreferably, at a cooling rate less than 15° C./s.

The cooling rate less than 1° C./s may cause ferrite to be formed duringthe cooling, thus undesirably lowering the strength of the hotpress-formed member. Accordingly, when performing the cooling, it may bepreferable that a lower limit of the cooling rate be limited to 1° C./s.

It may be preferable that hot forming be performed once the cooling iscompleted as described above. In the present disclosure, in order toimprove microcrack formation and crack propagation resistance, the hotforming temperature is controlled to 400-650° C.

Conventionally, it is preferable to increase the hot forming temperatureas much as possible in order to improve processability and reduce apress load. However, when a zinc or zinc alloy plating steel sheet isthermally treated at a high temperature, zinc may be dispersed andintroduced into grain boundaries, thus lowering the strength of grainboundaries of the base iron, and consequently, this may undesirablyfacilitate microcrack formation and crack propagation in the hotpress-formed member.

In light of the above, in the present disclosure, it may be preferablethat the hot forming be performed at a temperature of 650° C. or less,and not at conventional temperatures. When the hot forming temperatureis 650° C. or less, although it may vary depending on the heatingtemperature, the composition of the base iron, or the like, the strengthof grain boundaries may become greater than the strength of the baseiron, thus, it may be possible to improve microcrack formation andpropagation resistance in the hot press-formed member. However, if thehot forming temperature is too low, below 400° C., martensitic orbainitic transformation may occur prior to hot forming, causing hotforming properties to deteriorate.

Accordingly, in the present disclosure, it may be preferable that thehot forming be performed at a temperature of 400-650° C.

Immediately after performing the hot forming in the above-describedtemperature ranges, it may be preferable that cooling be performed to atemperature of 100° C. or less at a cooling rate of 1° C./s or more, toproduce a final hot press-formed member.

At this point, when the cooling rate is less than 1° C./s, ferrite maybe formed as a final microstructure, making it undesirably difficult toachieve ultra-high strength.

Conventionally, in case of 22MnB5 steel, the critical cooling rate is25° C./s or higher; however, in the present disclosure, manganese (Mn),a component of the steel composition, is added in an amount of 3.1% ormore, so even at a slow cooling rate of 1° C./s higher, ferrite may notbe formed. However, with regard to productivity, the cooling maypreferably be performed at a cooling rate of 5° C./s or higher, and morepreferably, at a cooling rate of 10° C./s or higher. The upper limit ofthe cooling rate is not limited to any particular value; however, withregard to equipment costs, it is preferable to limit the upper limit ofthe cooling rate to 1,000° C./s.

Once the cooling is completed, a hot press-formed member comprising 1-20area % of retained austenite, and remaining at least one of martensiteand bainite, as a microstructure, may be obtained.

In addition, a tempering step of heating the hot press-formed membercooled as described above to a temperature of 150-600° C. andmaintaining the hot press-formed member for 1-100,000 seconds may befurther comprised. This is to increase the stability of retainedaustenite, a microstructure of the hot press-formed member. When thecooling is completed after such a tempering treatment is performed, ahot press-formed member comprising 1-30 area % of retained austenite,and remaining at least one of tempered martensite and bainite, as amicrostructure, may be obtained.

When the tempering temperature is less than 150° C. or the maintainingtime is less than one second, it may be undesirably difficult to achievea sufficient stability of retained austenite phase. When the temperingtemperature exceeds 600° C., the strength of the hot press-formed membermay be markedly lowered, and the prior austenite grain size may grow tobe coarse, thus lowering impact toughness and causing heat-induceddeformation, and consequently, lowering precision in the shape of amember. Alternatively, the maintaining time exceeding 100,000 secondsmay markedly reduce the strength of the hot press-formed member and mayconsume time excessively, thus undesirably reducing the productivity ofthe member.

Hereinbelow, the present disclosure will be more fully described withreference to specific embodiments. However, it should be understood thatthe following embodiments are only examples and therefore, should not beused as an aid in determining the scope of the present disclosure, sincethe scope of the invention is determined by appended claims andreasonable equivalents thereof.

MODE FOR INVENTION Embodiment

A 40 mm-thick ingot, having the alloy composition shown in Table 1, wasmelted under vacuum and heated for one hour in a heating furnace at1,200° C., and thereafter, was finish hot rolled at 900° C. and coiledat 680° C. to produce hot rolled steel sheets having a final thicknessof 3 mm, wherein the furnace cooling temperature was 680° C.

Next, the hot-rolled steel sheets were pickled and cold-rolled at a coldreduction rate of 50% to produce cold-rolled steel sheets. Only when thehot-rolled steel sheet had a tensile strength of 1,500 MPa or greater,batch annealing was performed prior to the cold rolling. The batchannealing was performed by heating to 600° C. at a rate of 30° C./hour,maintaining for 10 hours, and cooling to room temperature at a coolingrate of 30° C./hour. Thereafter, the cold-rolled steel sheets werecontinuously annealed at 780° C., and were subjected to hot-dipgalvanization (GI) or galvannealing (GA) to produce coated steel sheets.Some of the cold-rolled steel sheets were subjected toelectrogalvanization (EG) to produce coated steel sheets. The GI, theGA, and the EG were performed under respective conventional conditions.

The coated steel sheets produced above were each produced as ahat-shaped hot pressed formed member as shown in FIG. 1 . The coatedsteel sheets were introduced into a heating furnace maintained under airatmosphere, and were heated to a maximum target temperature andmaintained for a target time, and then extracted from the heatingfurnace. Subsequently, the coated steel sheets were cooled to a hotforming temperature, and then hot formed and rapid cooled at the hotforming temperature to produce hat-shaped hot press-formed members. Theconditions for the heating, cooling, and hot forming are shown in Table2.

Tensile testing, microstructure observation, and XRD and GDS analyseswere performed using respective hot press-formed member specimens. Thetensile testing was performed at a testing rate of 10 mm/min using a JIS5 test piece, and phase fractions of microstructures were calculatedfrom the following equation using the integrated intensities of retainedaustenite peaks (γ) and martensite peaks obtained from the Cu targetX-ray diffraction analysis, and the results are shown in Table 2.

${V{\gamma(\%)}} = {\frac{\left\lbrack \frac{1}{{2.429\left( \frac{1\alpha^{200}}{1\gamma^{200}} \right)} + 1} \right\rbrack + \left\lbrack \frac{1}{{1.279\left( \frac{1\alpha^{200}}{1\gamma^{220}} \right)} + 1} \right\rbrack + \left\lbrack \frac{1}{{1.464\left( \frac{1\alpha^{200}}{1\gamma^{311}} \right)} + 1} \right\rbrack + \text{ }\left\lbrack \frac{1}{{1.328\left( \frac{1\alpha^{211}}{1\gamma^{200}} \right)} + 1} \right\rbrack + \left\lbrack \frac{1}{{0.699\left( \frac{1\alpha^{211}}{1\gamma^{220}} \right)} + 1} \right\rbrack + \left\lbrack \frac{1}{{0.801\left( \frac{1\alpha^{211}}{1\gamma^{311}} \right)} + 1} \right\rbrack}{6} \times 100}$

After prior austenite grain boundaries were exposed by an etching methodusing hydrofluoric acid, a prior austenite grain size (PAGS) wascalculated as an average value of average grain sizes measured at 5different points at a ¼ point of thickness of each base iron by using animage analyzing program, and the results are shown in Table 2.

In addition, a cross-section of the plating layer at R=4-30 mm from thelower end of a curved surface of the hat-shaped hot press-formed memberwas observed using an optical microscope, and a maximum crack depthpenetrating a base iron from a boundary between the base iron and theplating layer was measured through an optical image analysis. Asrequested by the client, the specimens with a maximum crack depthexceeding 10 μm were determined to be unacceptable. To measure a Mn/Zncontent ratio in surface oxides, an average Mn/Zn content ratio valuewas obtained from 1 μm point from each surface layer by using a glowdischarge spectrometer (GDS), and the results are shown in Table 2.

Some of the hat-shaped hot press-formed members were introduced in aheating furnace preheated to a target temperature, and were tempered andair-cooled. The tempering conditions were as shown in Table 3.

As described above, mechanical properties and microstructure phasefractions (retained austenite) were measured by performing tensiletesting and XRD analyses in the same manner as the above, using thetempered hot press-formed member specimens, and the results are shown inTable 3.

TABLE 1 Alloy Composition (wt %) Steel C Si Mn P S Al N B Others Notes A0.18 0.20 3.8 0.015 0.0030 0.03 0.0033 0 0 Inventive Steel B 0.15 0.145.0 0.014 0.0025 0.03 0.0042 0 Ti: Inventive 0.03 Steel C 0.21 1.52 5.90.007 0.0011 0.02 0.0050 0.0035 Ti: Inventive 0.04 Steel D 0.20 0.15 6.70.009 0.0030 0.03 0.0050 0.0026 0 Inventive Steel E 0.25 1.51 6.8 0.0070.0015 0.01 0.0037 0 Mo: Inventive 0.1 Steel F 0.26 0.10 7.5 0.0170.0018 0.008 0.0062 0.0032 Ti: Inventive 0.02 Steel G 0.23 0.20 1.20.014 0.0024 0.03 0.0038 0.0025 Ti: Comparative 0.03 Steel

TABLE 2 Hot Forming Conditions Maxi- Heating Main- Cooling FormingMechanical Retained mum Tem- taining Rate Tem- Properties γ Mn/Zn Crackperature Time (° C./ perature TS El Fraction PAGS Content Depth SteelPlating (° C.) (sec) s) (° C.) (Mpa) (%) (%) (μm) Ratio (μm) Notes A-1GI 800 60 10.3 620 1583 7.9 3.9 6.9 0.30 7.2 Inventive Example 1 A-2 GI800 60 10.1 500 1547 7.2 3.3 6.8 0.32 3.5 Inventive Example 2 A-3 GI 80060 31.5 500 1554 7.3 3.4 6.8 0.32 3.4 Inventive Example 3 A-4 GA 800 6010.0 450 1490 8.3 3.4 6.2 0.31 0.0 Inventive Example 4 A-5 EG 800 60 9.7400 1465 7.6 3.8 6.3 0.34 0.2 Inventive Example 5 A-6 GI 800 60 10.8 7101662 7.8 3.1 6.3 0.28 22.3 Com- parative Example 1 A-7 GI 800 6 10.5 6901627 8.1 3.5 6.0 0.29 17.8 Com- parative Example 2 A-8 GI 800 1 10.6 5501592 7.8 3.5 6.2 0.09 10.6 Com- parative Example 3 B-1 GA 800 60 10.3500 1653 9.6 4.0 4.1 0.39 0.0 Inventive Example 6 C-1 EG 780 120 10.5500 2035 7.9 5.7 2.8 0.52 0.0 Inventive Example 7 D-1 GI 780 120 10.3500 2089 6.7 6.5 2.5 0.64 0.0 Inventive Example 8 E-1 EG 780 120 10.4500 2231 6.6 7.1 2.3 0.67 0.0 Inventive Example 9 F-1 GA 750 60 10.2 5002407 7.3 11.3 1.8 0.72 0.0 Inventive Example 10 G-1 GI 930 60 12.7 7501539 6.6 0.3 12.7 0.07 28.5 Com- parative Example 4 G-2 GI 930 60 11.5550 631 18.2 0.1 12.5 0.05 10.3 Com- parative Example 5

(In Table 1, TS represents tensile strength, and El representselongation. Also, in Table 1, fractions other than the fraction of aretained austenite phase are of one or more types selected from bainiteand martensite.)

TABLE 3 Tempering Conditions Re- Tem- Mechanical tained per- Propertiesγ ature Time YS TS El Fraction Steel (° C.) (sec) (MPa) (MPa) (%) (% )Notes A-3 A-3-1 170 1800 1342 1581 9.4 4.3 Embodi- ment 11 B-1 B-1-1 1701800 1311 1564 10.4 7.1 Embodi- ment 12 C-1 C-1-1 200 30 983 1931 10.56.4 Embodi- ment 13 C-1-2 300 30 1260 1770 11.7 8.1 Embodi- ment 14C-1-3 400 30 1383 1572 12.9 12.8 Embodi- ment 15 C-1-4 500 30 1400 145215.8 17.8 Embodi- ment 16 D-1 D-1-1 200 30 989 1917 10.8 6.8 Embodi-ment 17 D-1-2 300 30 1240 1730 11.2 7.5 Embodi- ment 18 D-1-3 400 301239 1569 13.7 14.9 Embodi- ment 19 D-1-4 500 30 1231 1435 15.6 17.2Embodi- ment 20 D-1-5 600 30 1072 1279 18.4 19.8 Embodi- ment 21 E-1E-1-1 200 300 921 2052 11.9 7.8 Embodi- ment 22 E-1-2 300 300 1147 183613.4 13.6 Embodi- ment 23 E-1-3 400 30 1244 1671 14.9 15.6 Embodi- ment24 E-1-4 500 30 1351 1568 18.9 20.7 Embodi- ment 25 F-1 F-1-1 500 301180 1423 12.6 23.9 Embodi- ment 26 G-1 G-1-1 200 1800 1162 1482 6.9 0.3Com- parative Example 6 G-1-2 250 1800 1154 1425 7.1 0.4 Com- parativeExample 7 G-1-3 300 1800 1185 1351 3.1 0.5 Com- parative Example 8 G-1-4400 1800 1060 1162 3.1 0.6 Com- parative Example 9 G-1-5 500 1800 842903 9.1 0.9 Com- parative Example 10

(In Table 1, YS represents yield strength, TS represents tensilestrength, and El represents elongation. Also, in Table 1, fractionsother than the fraction of a retained austenite phase are of one or moretypes selected from bainite and tempered martensite.)

Steel G in Table 1 has the alloy composition of 22MnB5 steel, and has alow content of manganese (Mn) of 1.2%. Meanwhile, Steels A to F all havea content of manganese (Mn) in the range of 3.1-8.0%, all satisfying thealloy composition of the present disclosure.

As shown in Table 2, Steels A-1 to A-5 are Inventive Examples satisfyingthe hot press forming conditions proposed in the present disclosure, allof which have a retained austenite phase fraction of 1% or more, a PAGSof 10 μm or less, a Mn/Zn content ratio of 0.1 or more, and a maximumcrack depth of 10 μm or less. Also, these Inventive Examples haveachieved excellent ductility as well as ultra-high strength.

Steels A-6 and A-7 are the cases with a high hot forming temperature of710° C., and maximum crack depths exceeding 10 μm were observed evenwhen the retained austenite phase fraction was 1% or more. Steel A-8 isthe case with too short a maintaining time after heating, and it failedto achieve a Mn/Zn content ratio of 0.1 or more, thus, the maximum crackdepth exceeded 10 μm.

Steels B-1, C-1, D-1, E-1, and F-1 all have satisfied the hot pressforming conditions proposed in the present disclosure, and thus, allhave a retained austenite phase fraction of 1% or more, a PAGS of 10 μmor less, and a Mn/Zn content ratio of 0.1 or more, and had no observablemicrocracks. In addition, these Inventive Examples have achievedexcellent ductility as well as ultra-high strength.

However, Steel G-1 has, not only an insufficient alloy composition, butalso, due to a high hot forming temperature, an insufficiently formedretained austenite phase, a coarse PAGS, and a Mn/Zn content ratio ofless than 0.1, and thus, an extremely large maximum crack depth of 28.5μm was observed. In the case of Steel G-2, although the hot formingtemperature satisfied the present disclosure, it had a coarse PAGS and aMn/Zn content ratio of less than 0.1, and thus, the maximum crack depthexceeded 10 μm, and the tensile strength was 631 MPa, thus failing toachieve ultra-high strength.

FIG. 2 shows microcracks observed in hot press-formed members of SteelB-1 (Inventive Example 6) and Steel A-6 (Comparative Example 1).

Although in Inventive Example 6, a microcrack penetrating the base ironwas hardly observed, however, in Comparative Example 1, a crackpenetrating the base iron from the plating layer exceeded 10 μm.

As shown in Table 3, tempering was performed on produced hotpress-formed members, and in Inventive Examples 11 to 26 (A-1-1 toF-1-1), the retained austenite phase was sufficiently formed after thetempering, and thus, ductility was further improved, and yield strengthwas high.

However, in Comparative Examples 6 to 9 (G-1-1 to G-1-4), even after thetempering, the retained austenite phase was not sufficiently formed,thus resulting in inferior ductility, and Comparative Example 10 (G-1-5)failed to achieve ultra-high strength.

1. A hot press-formed member comprising: a base steel sheet; and aplating layer comprising Zn or Zn alloy on the base steel sheet, whereina Mn(wt %)/Zn(wt %) content ratio measured at a point 1 μm from asurface of the plating layer is 0.1 or more, wherein a maximum crackdepth penetrating the base steel sheet from an interface between thebase steel sheet and the plating layer is 10 μm or less.
 2. The hotpress-formed member of claim 1, wherein a microstructure of the hotpress-formed member comprises 1-30 area % of retained austenite.
 3. Thehot press-formed member of claim 1, wherein the microstructure of thehot press-formed member comprises 1-20 area % of retained austenite. 4.The hot press-formed member of claim 1, wherein the microstructure ofthe hot press-formed member comprises at least one of martensite andbainite, or at least one of tempered martensite and bainite.
 5. The hotpress-formed member of claim 1, wherein a prior austenite grain size ofthe hot press-formed member is 10 μm or less.
 6. The hot press-formedmember of claim 1, wherein a prior austenite grain size of the hotpress-formed member is 7 μm or less.
 7. The hot press-formed member ofclaim 1, wherein a prior austenite grain size of the hot press-formedmember is 5 μm or less.
 8. The hot press-formed member of claim 1,wherein the hot press-formed member comprises a metal coating comprisingFe or Ni between the base steel sheet and plating layer.
 9. The hotpress-formed member of claim 1, wherein the Mn(wt %)/Zn(wt %) contentratio is measured in an oxide layer of the plating layer.
 10. The hotpress-formed member of claim 1, wherein the base steel sheet comprises,by wt %, carbon (C): 0.08-0.30%, silicon (Si): 0.01-2.0%, manganese(Mn): 3.1-8.0%, aluminum (Al): 0.001-0.5%, phosphorus (P): 0.001-0.05%,sulfur (S): 0.0001-0.02%, nitrogen (N): 0.02% or less, and a balance ofiron (Fe) and other impurities.
 11. The hot press-formed member of claim1, wherein the base steel sheet further comprises at least one selectedfrom the group consisting of, by wt %, (1) at least one of chromium (Cr)and molybdenum (Mo), a total content thereof being 0.001-2.0%; (2) atleast one of titanium (Ti), niobium (Nb), and vanadium (V), a totalcontent thereof being 0.001-0.2%; (3) at least one of copper (Cu) andnickel (Ni), a total content thereof being 0.005-2.0%; and (4) Boron(B): 0.0001-0.01%.